1. Field of the Invention
The present invention relates to a film forming method and a film forming apparatus that forms an insulating film onto a semiconductor wafer in manufacturing semiconductor devices.
2. Description of the Related Art
When a semiconductor device is produced, an inter-layer insulating film is formed by, for example, an SOD (Spin on Dielectric) system. In the SOD system, a coating film is spin-coated on a semiconductor wafer (hereinafter referred to as a “wafer”) and an inter-layer insulating film is formed by performing processes like a physical or a chemical process such as baking and the like.
In a baking process, the process is performed stepwise under an environment according to the purpose. For example, in the first baking process, solvents like thinner, and the like, contained in the insulating film is cause to evaporate, then, by performing the second baking process at a higher temperature than the first baking process, the molecular frame of the insulating film is formed. In the end, the insulating film is burned to harden by “curing process” (hardening process), and the process of forming the insulating film is completed.
Incidentally, in the second baking process, for example, the wafer is heated at a temperature of less than 200° C. in an atmosphere of low oxygen concentration created by reducing pressure from the atmospheric pressure, so that the oxidation of the wafer is restrained while forming a frame of the insulating film. However, even under such atmospheric circumstance there are some cases that the oxidation may occur caused by desorption of a “porogen” in the insulating film. Here, “porogen” is a volatile substance used to cause the insulating film to be porous. As explained, if the wafer is oxidized in the second baking process, the curing process is to be performed without forming a frame of the insulating film in a good condition, therefore, formation of an insulating film of high quality becomes difficult.
In other words, primarily the stepwise heating process is aimed to forming the frame without oxidizing thereof in the second baking process, then accelerates the oxidation in the second baking process and forming an insulating film of high quality in the curing process. However the problem is that during the second baking process, desorption of the porogen from the insulating film occurs causing the oxidation of the film. As a result, there is a problem that porosity of the insulating film formed with the curing process becomes, for example, 70% or lower, therefore, the insulating film cannot be formed in a desired low dielectric constant.